The present invention relates to a new and improved method of operating a continuous casting installation with high capacity or throughput and also pertains to a new and improved construction of continuous casting installation for the performance of the aforesaid method.
During the continuous casting of steel, the strand emanating from the essentially vertically arranged continuous casting mold, and which strand possesses an outer shell or skin and a long liquid core, normally is guided and simultaneously cooled at a roller apron along a desired path of travel into a horizontal path of travel. By means of a withdrawal and straightening apparatus the strand is conveyed and straightened. The ferrostatic pressure acting upon the shell of the strand is taken-up by the rollers.
In the case of continuous casting installations operating at high throughput, that is to say, with continuous casting speeds exceeding 1 meter per minute for the casting of large slab cross-sections, there are required rollers of large diameter for supporting the forces acting upon the strand shell owing to the ferrostatic pressure. Thus, at the region following the continuous casting mold, that is to say, at the region of the strand which still has a thin outer shell or skin, it is not possible to prevent bulging thereof owing to the large distances between the supports and the absence of longitudinal supports between the rolls or rollers. This bulging produces the well known metallurgical defects, such as fissures and the like, which can also lead to metal breakout. Furthermore, a large withdrawal force is necessary since such bulging portions of the strand again must be pressed back by the rollers to the adjusted rated value.
In order to avoid such bulging between the rollers with higher casting speeds, it is known in this particular field of technology to arrange cooling plates and/or cooling grids at the region of the still thin strand shell or skin.
According to an unpublished proposal, the drawbacks of the bulging phenomenon arising at strands with large cross-sections at continuous casting installations operating with high throughput or capacity, for instance with casting speeds in the order of 2 meters per minute and more, are intended to be avoided in that the surface of the strand between the mold and the location of the complete solidification of the strand is subjected to pressurized water vapor. There is required for this purpose a pressure compartment arranged about the strand. The water vapor is generated by spraying water onto the surface of the strand. Since the ferrostatic pressure changes at the curved portion, the pressure compartment is subdivided at this region or portion, so that it is possible to approximately adjust the counter-pressure corresponding to the momentary ferrostatic pressure.
When the machine or continuous casting installation is cold, i.e., during interruptions in the casting operation, generating the required vapor pressure is associated with difficulties. Maintaining the machine in a warm condition or pre-heating the machine is associated with considerable costs.
If there is chosen the procedure of pre-heating the casting machine, then the machine is not operationally ready at all times because pre-heating requires a considerable amount of time. During disturbances in the casting operation, when exchanging the ladle during sequential pours, or at the end of casting with the therewith required reduction in the casting speed, the pressure in the compartment varies, producing damage at the solidified marginal zone with inclusions of water or vapor throughout the liquid steel and thus promoting the danger of explosions.